New 4.6 "stumble"???

lforgue8 · Dec 21, 2007

Just drove my truck last night for the first time and when I try to accel quickly it stumbled and also on the highway going up a longer hill same stumble.......

Timing is set at factory 6 btdc and idle is set and throttle pot is set at factory......plugs are new and oem, wires are used but oem, new cap and rotor, new ign mod and coil,

Question is can the timing be set more advanced (say 8 btdc) or is the 3.9 cux ecu not up to the task of the 4.6? pretty sure i stated everything important.....

Thanks

oh crower cam too

ptschram · Dec 21, 2007

I'd look to vacuum advance first. This is one of those things where brute force troubleshooting is gonna be needed. While I suspect the ECU is probably maxed out, without seeing realtime oxygen sensor output and injector pulse width, I can't say any more.

Look at traditional things before you jump to the ECU. Fuel pressure, vac advance can, etc. As far as timing is concerned, you'll find they run much better advanced, but get poor fuel economy and ping horribly-something you do not want, especially if there is any risk of lean A/F ratio.

PT

lforgue8 · Dec 21, 2007

vac advance can is that the unit itself or do you mean the "ball"? I dont have the ball hooked up i assumed it was for the cruise

thanks again pt

92rrrandall · Dec 21, 2007

A 4.6 engine swap is going to require some tuning to get everything right. I have a 4.6 high performance chip if you want to try it, send me a message.

Assuming your new ignition system is all RRC parts?? If you have substituted aftermarket coil, multi pronged plugs etc...need to know.

Best overall ig timing on a 4.6 Rover V8 should be about 28BTDC. The RRC distibutors that I have tested have about 16degrees of mechanical advance. Which would normally mean that you could use 12BTDC with the stock distributor. Of course there are many variables which would require retarding the timing.

You should use a vacuum gage to test for any indication of anything other than a perfectly running engine. This is one of the best diagnostic tools for quickly figuring out WTF is wrong.

When a TPS starts to die, it will usually cause a stumble on accelleration. Subtitute another TPS and see what happens.

Check the fuel pressure. Maybe it is weak, and when given the task of accellerating a 4.6, worn out pump can't handle it.

Which Crower cam part# did you install??

Randall

ptschram · Dec 21, 2007

lforgue8 said:
vac advance can is that the unit itself or do you mean the "ball"? I dont have the ball hooked up i assumed it was for the cruise

thanks again pt

The can on the distributor. The ball is a reservoir for the HeVAC vacuum stuff. The cruise is completely independent of engine vacuum.

Too much static ignition advance will do very bad things to an engine that may be running lean. Until we have an indication that the A/F ratio is not too far out of line, don't advance the ignition too far. Lean condition plus preignition=holes in pistons!

lforgue8 · Dec 21, 2007

ptschram said:
The can on the distributor. The ball is a reservoir for the HeVAC vacuum stuff. The cruise is completely independent of engine vacuum.

Too much static ignition advance will do very bad things to an engine that may be running lean. Until we have an indication that the A/F ratio is not too far out of line, don't advance the ignition too far. Lean condition plus preignition=holes in pistons!

Ill have to wait till i can get the truck to work to use my dvom and chk volts of the 02s to see if they are sweeping correct. the o2s are new spliced in universals and im sure i got the signal wire correct not 100% on the htr circuit but that shouldnt effect drivability should it?

ill leave the ign timing were it is now and chk the vac advance unit(looks new but i didnt replace it) mech advance is good and springs are tight to return after "twisted"

ill try and tackle some stuff tonight

ptschram · Dec 21, 2007

I am more concerned about where the oxygen sensor output indicates the A/F ratio is, rather than confirming they are working. The Lucas EMS can only control the A/F ratio in a fairly narrow band (+/- 0.625 of lambda, IIRC). Without having an opportunity to see what the sensors are telling the EMS and what the EMS is telling the injectors to do, it's difficult to know how lean or rich it is running.

If we can look at the EMS inputs and outputs in real time, we can determine with a high degree of certainty if the EMS is adequately feeding the larger engine. In theory, the EMS shouldn't care what the displacement is, but merely respond to the input from the oxygen sensors.

I do not suggest advancing the ignition timing until we determine the adequacy of fueling. Confirming that the vacuum advance is working can be done without risk though. Suck on the hose (!) and look to see if the plate in the distributor moves.

ca_surveyor · Dec 28, 2007

92rrrandall,

Would you provide more info on the max timing for the 4.6. I sent you an email. I may have a dist(Mallory) that gives too much advance because I have to run about 2 degrees BTDC to not ping (36 degrees @ 3000 RPM).

92rrrandall · Dec 28, 2007

The Mallory dist was designed for the 3.5 engine. The Mallory dist has 24degrees advance and the 3.5 needs~36degrees total timing. So with the Mallory on a 3.5 engine I run a base timing of 12degreesBTDC.

Unfortunately you would have to set the base timing at 4degrees with a Mallory/4.6 combo. This is no good. I would recommend using the stock ignition off a 1994 thru 1997 LR for the 4.6 engine. As I stated in my previous post, this will allow 12 degrees base timing and 28degrees overall...much better.

You may be able to send off the Mallory to have the advance curve set to match the engine. There are companies which will do this, but I do not know who they are.

Also there are a lot of factors which will allow you to advance the timing somewhat. You can use a lower temperature thermostat(180F vs 192F). You can fabricate a cold air intake. Only use premium octane fuel. Use one step colder spark plug. Clean the pistons and cylinder heads. Reprogram the fuel map. Etc.

Randall

ca_surveyor · Dec 29, 2007

Randall,

Thanks for the info. I was thinking that might be what I am experiencing. I'll talk to Mallory and see what they can do. I am already running a lower temp thermostat and have a snorkle which should cool the air.

ca_surveyor · Jan 3, 2008

Randall,

I just got conformation from Mallory that the distributor is adjustable. I just ordered a recurve kit that allows for maxi timing adjustment along with recurving springs to fine tune. I will start with static and max adjustments, before playing with any of the spring weight adjustments.

I'll let you know how it turns out, and if you have any suggestions let me know. Thanks for your assistance this time and for your related help from over a year ago.

lforgue8 · Jan 7, 2008

You know how most times when you figure out a problem it's like...........Jeeze how stupid do i feel now!!

doing a pre ride check under truck and saw cyl 4 header pipe still had paint on it! to my surprise a quick check under the hood i found that injector unplugged....( from when i was testing to get the truck started).....plugged back in and this truck is now a true 4.6! Wow what a difference!

I guess next time ill sit back and just go over the real basics!

Thanks PT once again....Thanks Randall for you help may still be in touch with you though.

ca_surveyor · Jan 9, 2008

Randall,

I recieved my recurve kit and tore it apart today. The recurve kit that Mallory recommended (#29015) was the incorrect one. I have been in touch today and the correct one (#29014) will be shipped out.

I did make a protractor sheet and performed an adjustment which has allowed me to advance static timing to 10BTDC and a maximum advance to approx. of 26 degrees. Once I get the new kit I will fine tune the max and also start playing with curve springs. I noticed that with the reduction of max advance, one of the springs wasn't being used. I know finally have an idle.

92rrrandall · Jan 9, 2008

Luc
Don't feel bad. It is very common to have one stupid little problem that you can't figure out for a week after an engine is put together.

ca surveyor:
I have a recurve kit also. Haven't used it yet because I am OK with the stock distributor on 4.6 engines. Let me know how it ends up with the Mallory recurved.

Randall

lforgue8 · Feb 21, 2008

I know this is old thread but i have come up with a stumble again....havent driven truck in a while so thought i had it nailed down but.......

Now just thinking out loud here i have a tick when engine is idleing could the cam walking inand out cause the timing to fluctuate and cause my stumble? anybody hear of that?

Also the efi light now has come and but havent check it yet....Ive got universal o2's in there and not sure if i got the heater wires correct but i know i got the signal wire correct thats all that matters right?

Timing is spot on but i wil try another new cap and rotor maybe this weekend just trying to get ideas

Thanks again

ptschram · Feb 21, 2008

If the EFI light came on on a Lucas EMS, something is happening bad enough that it really confused the EMS. Cam walk will cause timing variation, but I wouldn't expect it to be that much of an issue.

Having real time data on the EMS would be of great value to us as we still don't know how well the EMS is responding to the increased displacement. I suspect it doesn't care as the ability to respond in some degree of real time is pretty much unique to the Rover EMS scheme. Our only real danger here is that the EMS has maxed out the injector pulses and still can't supply enough fuel to achieve stoichiometry. If this is the case, we have some options, some more attractive than others. Chipping, tuning injectors/fuel pressure, etc.

Let's find out what codes we have and go from there, but again, I'd really like to know what the EMS thinks of the larger engine.

Ron · Feb 21, 2008

Read the code. I would bet you have a faulty O2 sensor. Cutting and splicing to save $100 between two O2s is false economy in my view. Get some correct O2s (Assuming that is the code) and it should be fine. Also, I would recommend using a chip out fo a 4.2 LWB.

ptschram · Feb 21, 2008

I just looked at Luc's profile and see that this is apparently in an '88RRC...

Get rid of that ECU and find one from a 4.2. Those older ECUs are really prone to stupid stuff happening, not to mention that you have really increased displacement compared to the 3.5 you started with. I have three of those ECUs that I can't trust and only have for future Megasquirt projects.

If your A/C still works, you'll want to do some mods, but otherwise, they are plug and play.

lforgue8 · Feb 21, 2008

Ive got a 14cux 3.9 ecu from a 94 d1 in there right now can i just find a 4.2 chip and install that?..........but i will try to use t4 at work to get some realtime readings if possible

ptschram · Feb 21, 2008

lforgue8 said:
Ive got a 14cux 3.9 ecu from a 94 d1 in there right now can i just find a 4.2 chip and install that?..........but i will try to use t4 at work to get some realtime readings if possible

Your T-4 reads Lucas EMS? Mine doesn't, I have to use the Rovacom-lite on Lucas trucks.

Take a look at oxygen sensor outputs to see if they are either at -22 (full rich, which is what I expect), or if they are fluctuating around 0. If they are always on one side or the other of 0, the EMS is not effectively controlling A/F ratio and we have more work to do.

PT

New 4.6 "stumble"???

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